In conventional reciprocating-piston, internal-combustion engines the process of the evolution of exothermic energy (heat release) is accomplished by a flame traversing the combustion chamber. In a gasoline engine it is a turbulent flame propagating across the charge. In a diesel engine it is a diffusion flame which is usually established as an envelope around a cloud of evaporating fuel spray (the so-called group combustion mode). As a consequence, in both cases the spatial and temporal distribution of the specific exothermic power (rate of heat release per unit mass of the working substance), as well as the residence time of reacting particles in the zone of the most effective chemical activity (region of significant concentration of active radicals) are virtually beyond control. This is exacerbated by the fact that the expansion due to the deposition of the exothermic energy in the reacting medium tends to expel the reacting particles prematurely from this zone. The reason for this is that a close coupling between the exothermic region of chemical activity with the flame front is essential to assure a sufficiently high rate of flame propagation so that combustion is completed within the relatively short time interval required for proper operation of the engine.
In conventional premixed charge or gasoline (Otto) engines combustion is initiated by forming a flame kernel whose front thereupon sweeps across the working substance. The important point is that after ignition takes place, the combustion process spreads through the head space at its own natural speed, essentially beyond any further control. The specific exothermic power as well as the residence time of the reacting species in the zone of the essential chemical activity are virtually uncontrolled.
In conventional non-premixed charge or diesel engines, liquid fuel is injected into piston-compressed air at an appreciable inlet velocity. Upon entering the combustion chamber, the fuel is atomized into a set of droplets whose number density is high enough to form a cloud of sufficiently densely spaced fuel droplets for the flame to become established as an envelope around it. Its front is then driven across the compressed air charge as a consequence of the momentum imparted upon the spray in the course of its formation by the injector, an action leading often to the detrimental effects of fuel wetted cylinder walls.
The establishment of the front of a diffusion flame front as an envelope of a spray is technically referred to as its group combustion mode. Under such circumstances oxygen is completely depleted inside the flame envelope while fuel is fully consumed at the front. As a consequence, maximum temperature the fuel is capable of reaching by combustion in air, is actually achieved at the flame front, stabilizing the process of combustion. This maximizes, however, the formation of nitric oxide and, in approaching this high temperature zone in the absence of oxygen, fuel is pyrolized to generate soot. Moreover, as a consequence of imperfections due to the relatively narrow zone of the exothermic power pulse associated with the essential chemical activity concentration at the front, optimum conditions are attained for the generation of carbon monoxide and the formation of a residue of unburnt hydrocarbons. In essence then, the combustion system acquires automatically the most favorable conditions for the generation of all the known pollutants.
To make matters worse, in order to assure good contact of fuel with air, using the conventional system of a single injector per cylinder, one has to rely on the momentum of the spray in order to drive the flame across the compressed air charge. Created thus is the familiar noise of diesel engines and the concomitant tendency to knock, creating the demand for fuels of a relatively high cetane number, that is fuels that auto-ignite relatively fast to keep up with the flow rate at which they are injected into the combustion chamber.